Plant for feeding alternating voltage with reduced losses

ABSTRACT

A plant for feeding alternative voltage between places located remotely through a long first line includes a feeding arrangement for feeding electric power to the first line. This arrangement has a second line conducting direct voltage and adapted to extend along the first line, and inverters arranged at locations along the extension of the lines. The inverters are connected between the direct voltage line and the alternating voltage line for converting direct voltage into alternating voltage which is fed to the first line.

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a plant for feeding alternating voltagebetween places remotely located through a long first line, whichcomprises a feeding arrangement for feeding electric power to the firstline.

The invention comprises all types of such plants, such as for examplefor transmitting alternating voltage through high voltage lines overlong distances from places where it is generated to consumer networks.“Places located remotely” and “long first line” mean that the distanceis at least in the order of kilometres, but lines of this type extend inpractice over hundreds of kilometres.

In order to illuminate the invention and the problem to be solvedthereby the case of such a plant for a line for feeding alternatingvoltage to railway vehicles will hereinafter be described, although theinvention, accordingly, is not at all restricted to this field of use,but the corresponding problem is also there in other possible fields ofuse of the invention.

In such plants in so-called railway supply a one-phase alternatingvoltage is fed to railway vehicles through said first line, and sincethe first line has a comparatively high impedance, both resistance andreactance, the power possible to transmit to the railway vehicles alongthe first line will be restricted as a consequence of the phase shiftbetween the current and the voltage, voltage drops over the line andlosses in the line.

Different solutions have been suggested for solving this problem with alimited transmittable power from the first line, which aim at reducingthe impedance in the first line. Among these, lines with double supplyand arrangement of different types of reinforcement lines running inparallel with the first line constituting the contact line may bementioned. Another way to attack this problem is to arrange alternatingvoltage transmission systems in parallel with a higher voltage than thevoltage of the first line. This means that the voltage is step-uptransformed to a higher level and led in parallel with the first line,which means considerably lower losses, in which the voltage may forexample be step-up transformed from about 16 kv in the first line to 130kV in the parallel line. The voltage is then step-down transformed atdifferent locations along the first line for feeding thereof. Completelyseparate high voltage transmission lines may so be used with aconnection to the first line through transformers uniformly distributedor a use of autotransformer systems may take place. One of thedisadvantages of this solution is the requirement of expensivetransformers. All these solutions are comparatively costly.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a plant of the typedefined in the introduction, which enables an achievement of a goodability to transmit electric power from the first line to loadstherealong in a simpler and by that less expensive way than through theplants already known discussed above.

This object is according to the invention obtained by providing saidarrangement with a second line conducting direct voltage and adapted toextend along the first line and inverters arranged at locations alongthe extension of the lines and connected between the direct voltage lineand the alternating voltage line for converting direct voltage intoalternating voltage fed therethrough to the first line.

The feeding of electric power between different locations along thefirst line may by providing the feeding arrangement with a lineconducting direct voltage take place through a direct current which willonly feel the resistive part of the second line and the impedance andthe voltage drop over the line will by that be lower. Furthermore, thecurrent in the direct voltage line may without any problem beconsiderably lower than in direct transmission over the first line,since a considerably higher, namely 2{square root over (2)} times,direct voltage may be transmitted on the direct voltage line than thevoltage level desired to be obtained on the first line without any needof any transformer for reducing the voltage to the level desired betweenthe inverter and the first line. This will result in a substantiallyincreased transmission capacity and reduced losses.

According to another preferred embodiment of the invention the feedingarrangement comprises a third line conducting alternating voltage andled to the first line for feeding electric power to the first line, anda rectifier is connected between the third and the second line. A supplyline conducting alternating voltage and emanating from a source forgeneration of electric power may by this in the plant according to theinvention in a conventional way be led to the first line for feedingpower thereto, in which, however, a rectifying takes place through therectifier for being able to provide an alternating voltage of adifferent type through an inverter, primarily with respect to the numberof phases and the frequency, to the first line, in which the directvoltage produced by the rectifier of the second line conducting directvoltage here is utilised for a direct voltage transmission in the secondline to locations where inverters are arranged and may transfer electricpower to the first line.

According to another preferred embodiment of the invention the feedingarrangement comprises a converter with a rectifier connected to thethird line, an inverter connected to the first line and an intermediatelink arranged therebetween, and the second line is connected to theintermediate link for feeding direct voltage along the first line. By inprinciple prolonging the direct voltage line present between a rectifierand an inverter in a conventional converter in this way a simple way totransmit electric power along a line for feeding alternating voltage isobtained while maintaining a good transmitting capacity from the firstline to possible loads and low losses along the very line.

According to another preferred embodiment of the invention the firstline is a contact line for feeding one-phase alternating voltage torailway vehicles. The invention is particularly well suited for thisapplication, in which it is important that sufficient power may betransferred to such a vehicle at different locations along the contactline.

According to another preferred embodiment of the invention theconnection between said inverter and the first line has no transformer,which keeps the costs for the plant on a low level and still enables acomparatively weak current with small losses in the second directvoltage transmission line.

According to another preferred embodiment of the invention the inverteris a three-level inverter, i.e. an inverter having a neutral point,whereby a better curve shape is obtained for the alternating voltageprovided to the first line by the inverter.

It is pointed out that the claim definitions “rectifier” and “inverter”are to be given a broad sense and mean that in the direction intended,such as from the third line to the second line, a transition fromalternating voltage to direct voltage and from direct voltage toalternating voltage, respectively, takes place, but this does notexclude the possibility to transmit power through these components inthe opposite direction, so that these then function as inverter andrectifier, respectively, instead, which will be the case when a railwayvehicle will brake.

Further advantages as well as advantageous features of the inventionwill appear from the following description and the other dependentclaims.

BRIEF DESCRIPTION OF THE DRAWING

With reference to the appended drawing, below follows a description ofpreferred embodiments of the invention cited as examples.

In the drawing:

FIG. 1 shows a schematic block diagram illustrating how a plantaccording to the invention according to a first preferred embodiment ofthe invention may look like,

FIG. 2 is a more detailed view of a part of the plant according to FIG.1, and

FIG. 3 is a part of a plant of the type shown in FIG. 1 according to asecond preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The principles of a plant according to the invention, which here isshown in the form of a plant for feeding alternating voltage to railwayvehicles, are schematically illustrated in FIG. 1. This plant has afirst line 1 in the form of a so-called contact line for feeding railwayvehicles by a one-phase alternating voltage, which typically may have aneffective value of for example 16 kV and a frequency of 16⅔ Hz. The line1 hangs at a suitable distance above the railway 2. The line 1 has aconsiderable impedance, usually about 0,3 Ω/km, which has an inductiveand a resistive part.

The plant has also a third line 3, which here leads a three-phasealternating voltage (another number of phases is also possible) and isarranged to supply the contact line with electric power. The third line3 is connected through a transformer 4 (see FIG. 2) for step-downtransforming the high voltage on the line 3 from for example 200 kV to16 kV to the input of each of two rectifiers 5 connected in series,schematically indicated and adapted to convert alternating voltage intoa direct voltage. One respective of two pole lines of the second line 6conducting direct voltage are connected to the outputs of the rectifier,and this direct voltage line is arranged to extend along the contactline 1. The midpoint of the rectifier outputs is connected to the rail2. At different locations along the direct voltage line 6 and by thatthe contact line 1 inverters 7 are arranged between the direct voltageline and the contact line for transmitting alternating voltage andcurrent to the contact line at these locations. The advantages ofarranging the direct voltage line 6 along the contact line in this wayfor transmitting through the direct voltage line over the longerdistances over which the contact line extends have been thoroughlydiscussed above.

More details of the plant according to the invention will now bedescribed with reference to FIG. 2. The rectifier 5 consists, asmentioned, of two halves 8, 9, and the midpoint therebetween isconnected to the rail 2. Both halves 8 and 9 of the rectifier are madeof so-called valves having a number of rectifying semi-conductorcomponents connected in series. This is conventional technique. Anintermediate link is connected to the output of the rectifier and isformed by capacitors and inductance coils. Harmonic or overtone filters13-16 are formed in this way for eliminating power pulsations of theone-phase side. Each inverter 7 consists in conventional way of twoelectronic switches 11, which connect the phase outlet 12 either to plusor to minus. The electronic switches are in practice constituted by anumber of semi-conductor components of turn-off type, such as IGsTs,connected in series and a diode connected in anti-parallel with each ofthem. The phase outlet 12 is connected to the contact line 1 through aLCL-filter, i.e. two inductors and one capacitor and accordingly withoutany intermediate transformer.

Since the voltage of the intermediate link 10 has to be higher than thepeak-to-peak value for the alternating voltage to be generated to thecontact line, for example in the case of a voltage in the contact lineof 16,5 kv in the case of a Swedish system a voltage on the intermediatelink of at least 2{square root over (2)}×16,5 kV=46,7 kV has to be used.This means in its turn that the current will be at least 2,8 times lowerin the line 6 than in direct transmission from the contact line and inaddition a direct current, which means that it is only the resistivepart of the direct voltage intermediate link that gives rise to thevoltage drop. This means, as declared above, a substantially increasedtransmission capacity and reduced losses.

Another way to construct the inverter of a plant according to theinvention is illustrated in FIG. 3, in which this here is designed as aso-called neutral point(NPC)-inverter having three levels. The design ofsuch an inverter is conventional technique. The arrangement of aninverter of this type means that the voltage jumps to be taken by theinductor of the one-phase filter will be halved, which results in abetter curve shape.

The invention is of course not in any way restricted to the preferredembodiments described above, but many possibilities to modificationsthereof would be apparent to a man skilled in the art, without departingfrom the basic idea of the invention.

The number of inverters of the plant could for example be another thanshown in the figures, and an inverter would usually be arranged inconnection with the rectifier in a so-called converter station.

As already mentioned, the different alternating voltage lines could haveanother number of phases and other voltage levels and frequencies thanmentioned above.

What is claimed is:
 1. A plant for feeding alternating voltage betweenplaces remotely located through a long first line comprising: a feedingarrangement for feeding electric power to the first line, wherein saidfeeding arrangement includes a second line conducting direct voltage andadapted to extend along the first line, and a plurality of invertersarranged at locations along the extension of the lines and connectedbetween the direct voltage line and the alternating voltage line, saidplurality of inverters converting direct voltage into alternatingvoltage fed therethrough to the first line.
 2. A plant according toclaim 1, wherein the feeding arrangement further comprises: a third lineconducting alternating voltage connected to the first line for feedingelectric power to the first line, and a rectifier connected between thethird line and the second line.
 3. A plant according to claim 2, whereinthe feeding arrangement further comprises: a converter with a rectifierconnected to the third line, an inverter connected to the first line,and an intermediate link arranged therebetween, wherein the second lineis connected to the intermediate link to feed direct voltage along thefirst line.
 4. A plant according to claim 3, wherein said inverter isarranged in a station close to the rectifier and is connected to thefirst line, and at least one additional inverter is connected betweenthe first line and the second line at a substantial distance from saidinverter connected to the first line.
 5. A plant according to claim 1,wherein the first line is a contact line for feeding a one-phasealternating voltage to railway vehicles.
 6. A plant according to claim3, wherein the third line is a three-phase alternating voltage line. 7.A plant according to claim 6, wherein a transformer is arranged betweenthe third line and said rectifier for step-down transforming thevoltage, before rectifying thereof.
 8. A plant according to claim 1,wherein said inverter has switches in the form of semiconductorcomponents of a turn off type, and said plant further comprises acontrol arrangement for controlling the switches of the inverter andobtaining a pulse width modulation pattern.
 9. A plant according toclaim 1, wherein the connection between said inverter and the first linehas no transformer.
 10. A plant according to claim 9, wherein a filterwith an inductor and a capacitor for eliminating harmonics is arrangedin the connection between the inverter and the first line.
 11. A plantaccording to claim 1, wherein the inverter is a three-level inverterhaving a neutral point.
 12. A plant according to claim 2, wherein saidinverter is adapted to convert said direct voltage into an alternatingvoltage of the first line with another frequency that a frequency of thealternating voltage of the third line.